This project is designed to determine the minimal dose of physical activity necessary to improve cardiovascular (CV) health. More specifically, this 4-year investigation will evaluate and aerobically train 500 healthy men and women, 30-65 years of age. Both the minimal and optimal intensity and frequency of exercise necessary to elicit significant improvements in cardiovascular health will be determined. The plans also include assessment of the effect of specific doses of exercise on the reduction in systolic and diastolic blood pressure and increase in HDL-cholesterol (primary outcomes), in addition to changes in levels of physical activity, aerobic fitness (VO2 max), body composition (% fat and waist-to-hip ratio), triglycerides, and LDL-cholesterol (secondary outcomes). Psychological variables (depression, anxiety, anger, and perceived stress) and health-related quality of life will also be evaluated, and the important issue of adherence to exercise will be examined with respect to the effect of dose of exercise. Subjects will be randomized into one of 5 groups in a 2x2 factorial design plus a usual care control group (n=100 in each of 5 groups). Subjects will train at a moderate (45-55%) or high (65-75% of maximum heart rate reserve) intensity, and at a frequency of 3-4 or 5-7 days/wk (viz., Group 1, 65-75% @ 5-7 d/wk; Group 2, 65-75% @ 3-4 d/wk; Group 3, 45-55% @ 5-7 d/wk; Group 4, 45-55% @ 3-4 d/wk; and, Group 5, usual care control). Controls will receive advice from a physician to increase their physical activity. The subjects in Groups 1-4 will participate in an exercise intervention using walking as the mode of training for a total of 24 months. A lifestyle model of physical activity will be used, in that subjects can accumulate exercise minutes during the day in 10-min increments or more continuously (for a total duration of 30 min/day), and training will be conducted where it is most convenient for the subject (i.e. at home, work or some combination thereof). Tests will occur at baseline (0), 12, and 24 months of the intervention. The application posits that Groups 1-4 will show a significant reduction in systolic and diastolic blood pressure and an increase in HDL-cholesterol compared to the usual care control group. It further suggests that there will be significant main effects for both frequency and intensity, with the higher levels of each producing greater improvement on the major outcome variables compared to the lower levels of each dose. The study is also planned to determine the minimal dose response to exercise necessary to elicit significant improvement in cardiovascular and behavioral health factors. Finally, the project includes examination of the effects of age and level of initial test values on outcome variables to determine if their interactions affect the results of the intervention. The results from the proposed study will provide important data regarding the dose of exercise required to improve CV health, as described both by the NIH Consensus Conference on Physical Activity and CV Health and by the recent Report of the Surgeon General.